The present invention relates to the design of tension legs, i.e. tendons for Tension Leg Platforms (TLP) and method for transport and installation and removal/replacement of tendons and similar slender bodies in a body of water.
There are two main types of tendons:
Rods made of composite or fiber materials, and
hollow circular pipes welded into sections where the design seeks to utilize the buoyancy as much as possible to reduce submerged weight of the tendon in order to lessen the size of the platform that carries the weight of the tendons.
A major feature of the latter concept is that air at atmospheric pressure is present in the interior of the pipes in a tendon. Hence, when installed the tubular pipes/tendon are exposed to outer hydrostatic pressure that is an disadvantageous buckling-type loading calling for increasing the wall thickness to diameter ratio, i.e. increasing wall thickness and decreasing diameter. With increasing water depth the ratio increases and consequently the submerged weight of the tendon increases. The final consequence is that the platform""s displacement must be increased that again causes larger loads on the tendons etc. This is even more pronounced for deep-water TLP tendons.
In detail two concepts of these tendons are known:
Stepped diameter tendon seeking to employ as large diameter as possible to resist the increasing hydrostatic pressure thus to achieve as much buoyancy as possible. In deeper waters it is not feasible to achieve a desirable or optimum design, namely neutrally or positively buoyant tendon. Further, this solution requires transition sections when the diameter is changed. Such transitions attract stresses causing material fatigue.
Uniform diameter tendon eliminating the fatigue stresses on the penalty of small diameter thus large loads on the platform with the associated negative consequences. The maximum achievable diameter is limited to that needed at the lower end of the tendon.
Two main transport and installation concepts are used for tendons made of hollow cylinders:
Prefabricated sections of a tendon equipped with expensive and fatigue prone connectors, are transported on deck of a vessel to installation site where these sections are handed over to a crane vessel. The crane vessel upends and connects the sections into a vertically hanging string and adds sections until the entire tendon has been completed. This is an expensive solution due to expensive vessel and connectors.
Tendon prefabricated in full length at a shore-based facility is launched into water while temporary buoyancy elements are being attached to the tendon in order to keep the tendon floating. Thereafter the tendon is towed in surface to the installation site where, by applying different more or less cumbersome methods, the tendon is upended by reduction or removal of the temporary buoyancy. Two major drawbacks are associated with such methods: (a) When towed in waves, the tendon is exposed to fatigue stresses; and (b) Large risk is associated with the use of external temporary elements, as experienced in practice. Another disadvantage is associated with these methods that is non-reversibility of some of operations. Upon negative experience from practical applications the industry is hesitant for further use of these methods.
Other slender bodies such as bottom pipelines and flowline bundles have been towed to offshore sites for decades. In addition to the surface tow method mentioned above, the tow of such bodies has been carried out on bottom, elevated off bottom or at mid depth where the negatively buoyant body was suspended and tensioned by two tow vessels. Also these methods are associated with disadvantages. The former two can be used in special cases only when depth, bottom conditions and integration into a seabed system allow. The latter is characterized by demanding control, large loads involved and lack of facility to bring the body to surface in case of contingency or for planned operations such as connection with other sections or preparation for installation.
In accordance with the present invention the tendon is designed and made, in traditional fashion, of hollow cylindrical sections such as pipes that are connected into a continuous string. However, the interior of the string is divided into one or several compartments. The optimum number of compartments depends on water depth at installation area, typically 6 to 10. Before installation of the tendon each of the compartments is pressurized by gas, e.g. nitrogen or dried air, to a pressure that is close or equal to ambient water pressure at the depth where the compartment will be found after completed installation. Hence, the undesirable external stresses generating buckling loads are eliminated or significantly reduced during the time when the tendon is in use. This opens for the possibility to increase diameter, thus increase the inherent buoyancy of the tendon without the need of larger wall thickness, as it would be required if the traditional design would be applied. Therefore, the designer can optimize the diameter in order build-in desirable buoyancy for transport, installation, operation and finally removal. At the same time pipes of standard dimensions can be used and material saving is achieved, typically 30% weight reduction compared to standard design of tendons for a deepwater platform (i.e. platform at more than 3.000 ft water depth).
The tendon can be designed with uniform diameter over its entire length in order to eliminate locations exposed to fatigue stresses. In such a case the tendon upon completion of pressurization would have stepwise increasing net buoyancy when floating in surface because of decreasing weight of pressurized gas in the compartments, the bottom compartments being the heaviest thus least buoyant. Since the transport and installation method requires uniformly distributed net buoyancy the excessive buoyancy is counterweighted by ballast added.
The tendon can also be designed with stepped diameter so that the above-explained need for ballast is eliminated. In such a case attention is paid to design of details of the transitions so that the fatigue loads are eliminated or significantly reduced.
Some of the lower compartments can be flooded after installation connection to the platform. The weigth of flooded water represents reduction of upward tension loads (typically several hundred tons) on the anchor, and thus reduces the required size/weigth of the anchor.
Identically with the state of the art installation technology also here the tendons are towed in horizontal position and at the installation site upended and connected to pre-installed anchor. The present inventive tow method is of general applicability and overcomes all major disadvantages of the existing methods briefly described above without introducing shortcomings of significance for time, costs or safety. The present method is characterized as follows:
At initiation and termination of the tow or in contingent situation the towed slender body such as a flowline bundle, a pipeline section or a TLP tendon attains a safe and comfortable position floating in the surface.
During tow the slender body is submerged to desirable depth to avoid fatigue loads from waves.
Physical laws prevent, for all practical applications, possibility that the body towed in submerged position would collide with seabed in unpredicted or unexpected shallow water.
The inventive installation method for tendons and similar structures is integrated with the present tow method in the sense that the tow vessels perform the upending and rough positioning as a natural continuation of the tow and without need for re-rigging or other interventions. When upended and positioned to target area close to the pre-installed anchor, the tendon is pulled to vertical or side entry bottom connector on the anchor by means of moderate forces generated in simple rigging.
Another advantageous feature achieved by the invention is simple removal of the tendon, intact or with one accidentally flooded compartment. The inventive removal method for tendons is facilitated by the inherent properties of the tendon in accordance with the invention, namely:
The compartmentation of the interior and pressurization of gas inside the compartments limit the amount of water that can leak into the interior.
The pressurized gas inside the compartments enables to displace all or parts of the flooded water hence to ensure, in most practical instances, that the tendon can regain positive buoyancy that simplifies the retrieval and tow of the tendon.
From the design of the tendon point of view the most important advantage achieved is reduced consumption of material hence lower price. This aggregated advantage is a result of the following:
Reduced or eliminated loading from ambient water pressure.
Facilitated use of pipe sections of standard dimensions and materials.
Further, the design allows for greater water depth in which metallic tendons can effectively be used. Moreover, the inherent buoyancy of the tendon lessens the size of temporary buoyancy tanks required to keep tension in the tendon before installation of the platform is completed and tension can be generated by the platform itself. Finally, the design allows for flooding of dedicated compartments after installation of the platform with the aim of reducing loads on the anchors.
From the transport of the tendon or any other similar object point of view the most important advantages achieved are as follows:
Increased safety due to inherent fail-to-safe ability in the sense that in case of failure the object floats up to a stable position in surface and the object is prevented from collision with seabed in case of unidentified shallows.
Reduced fatigue loading due to the fact that the object is transported submerged below wave zone.
Eliminated possibility of unintentional collision with seabed during tow.
Reduced transport cost resulting from eliminated or reduced need for temporary buoyancy elements or floats that are commonly used for achieving desired buoyancy for transport.
From the installation of the tendon point of view, i.e. upending, positioning and connecting, the most important advantages are reduced price due to simple installation gear and applicability of inexpensive vessels, reversibility and easy control of all operations.
From the removal/replacement point of view the most important advantages is the simplicity of transferring the negatively buoyant tendon from its vertical position to horizontal position in which it is floating in surface, i.e. positively buoyant and ready for tow to the receipt destination. This simplicity is achieved in most of the expected instances when removal of the tendon is required.